Nghiên cứu tổng hợp tio2 và khả năng ứng dụng để xử lý cr6+ trong nước thải = reseach on synthesis of tio2 and application for cr6+ treatment in wastewater

When rutile type titanium oxide and anatase type titanium oxide are irradiated with light of 413 nm or lower, or 388 nm or lower, respectively, valence band electrons move up to the cond

TRƯỜNG ĐẠI HỌC BÁCH KHOA HÀ NỘI

NGUYỄN VĂN CHÚC

NGHIÊN CỨU TỔNG HỢP TIO2 VÀ KHẢ NĂNG ỨNG DỤNG ĐỂ XỬ LÝ SR6+ TRONG NƯỚC THẢI

LUẬN VĂN THẠC SĨ KHOA HỌC

NGƯỜI HƯỚNG DẪN : TS NGUYỄN HỒNG LIÊN

HÀ NỘI – 2010

future career as an expert in petrochemical by all my efforts and brain power

To my supervisors, I would like to acknowledge and extend my heartfelt gratitude to

Doctor Nguyen Hong Lien and Associated Professor, Doctor Le Minh Thang,

lecturers of the Department of organic and petrochemical technology Within their petrochemical course, I luckily found my interest in petrochemical technology, and then under their guidance I continued my study in petrochemical technology in my thesis Indeed, I am deeply in debt of their endless patience for their correcting tiny mistakes in

my thesis and priceless encouragement that enables me to complete my thesis in due time

Last but not less, from deep inside, it is hard to find a suitable word to send my deep thank and express my strong love for my family, my mother, my brothers, my sisters and

my girl friend for both their physical and spiritual support Although I have to live far from them, I always feel fully their images and eternal love by my side It is the time I realize that the family is the most wonderful thing I luckily possess

To all, I wish you the best

Hanoi, October 2010 Student

Nguyen Van Chuc

TABLE OF CONTENTS

ACKNOWLEDGEMENT 1

TABLES IN THE THESIS 4

FIGURES IN THE THESIS 5

INTRODUCTION 7

CHAPTER I: LITERATURE REVIEW 8

I.1 General of wastewater 8

I.1.1 Water pollution 8

I.1.2 Water pollution categories 9

I.1.3 Causes of polluted water 10

I.1.4 Effects of polluted water 10

I.1.5 Control of water pollution 10

I.2 Cr(VI) treatment methods in wastewater 12

I.2.1 Ion exchange method 12

I.2.2 Electrochemistry method 13

I.2.3 Reduction-oxidation method and deposition method 13

I.3 TiO2 review 14

I.3.1 Occurrence 14

I.3.2 Physical and mechanical properties 14

I.3.3 Chemical properties 16

I.3.4 Applications 16

I.4 Mechanism of titanium oxide photocatalytic reactions 17

I.4.1 Band structure of semiconductors and band gap energy 17

I.4.2 Energy structure of titanium oxide and photoeffect 18

I.4.3 Effect of ultraviolet rays in activating titanium oxide 20

I.4.4 Decomposing power of titanium oxide photocatalyst 21

1.4.5 The mechanism of photo-reduction of Cr(VI) 23

I.5 Review of the used precursors in this thesis 24

I.5.1 TiCl4 24

I.5.2 Titanium isopropoxide (TTIP) 28

I 6 Literature review about using TiO2 as a photocatalyst for wastewater treatment 29

I.6.1 In Vietnam 29

I.6.2 In the world 30

I.7 The importance and direction of thesis 36

I.7.1 The importance of the thesis 36

I.7.2 The direction of thesis 36

CHAPTER II EXPERIMENTAL 37

II.1 Reagents and materials 37

II.2 Preparation of photocatalysts 37

II.2.1 Sol-gel method 37

II.2.2 Hydrolysis method 38

II.2.3 Impregnating method 39

II.3 The methods to determine the composition of chromium plating wastewater 40

II.3.1 pH meter 40

II.3.2 Atomic Absorption Spectroscopy (AAS) 41

II.4 Physico-Chemical experimental techniques 44

II.4.1 X-ray diffraction (XRD) 44

II.4.2 Scanning electron microscopy (SEM) 46

II.4.3 BET method for the determination of surface area 47

II.5 Catalytic activity test 49

II.5.1 Equipment description 49

II.5.2 The analysis of the composition of reaction solution 50

CHAPTER III RESULTS AND DISCUSSION 63

III.1 Composition of plating chromium wastewater 63

III.2 Physico chemical properties of synthesized catalysts 65

III.2.1 The specific surface areas (SSA) of synthesized catalysts 65

III.2.2 The phase composition of synthesized catalysts 66

III.3 Catalyst activity of synthesized catalysts 70

III.3.1 The influence solar illumination and the suport to the catalytic activity 70

III.3.2 The influence of pH of reaction solution to the catalytic activity 71

III.3.3 The influence of catalytic synthesis methods to the catalytic activity 72

III.3.4 The influence calcination temperature of catalyst to the atalytic activity 73

III.3.5 The influence of amount of TiO2 doped onto Al2O3 to the catalytic activity 74

III.3.5 The influence of concentration of ethanol to the catalytic activity 74

CONCLUSIONS 76

References 77

TABLES IN THE THESIS

No Name Page

1 Typical physical and mechanical properties of titania 15

7 The amount of TiO2 loading onto Al2O3 of synthesized catalysts 40

8 The concentration of the metallic ions in wastewater 64

10

Compositions of catalysts prepared by sol-gel, hydrolysis and

Impregnating method

69

12 The effects of UV and catalyst to conversion of Cr(VI) and COD 72

FIGURES IN THE THESIS

11 Illustrates how diffraction of X-rays by crystal planes allows one to derive

lattice by using Bragg relation

46

12 The interaction between the primary electron beam and the sample in an

electron microscope leads to a number of detectable signals 48

15 The calibration graph of Cr(VI) with 1,5 – diphenylcarbazide solution 56

16 Concentration of Cr(VI) in three reaction solutions 57

17 Concentration of Cr(VI) in reaction solution with and without

Flocculation

58

18 The XRD patterns of the TiO

20 The conversion of Cr(VI) deppended on pH of reaction solution 73

21 The conversion of Cr(VI) deppended on the catalytic synthesis

In this paper, the research topic focuses on the reduction of heavy metal concentrations by using TiO2 as a photocatalyst Cr(VI) is choosen as a poluted agent and ethanol as a hole scavenger The TiO2/Al2O3 system is synthesized by variety methods and many effect agents are tested The properties of the sythesized catalysts are investigated by the physico-chemical method The catalytic activities of the sythesized catalysts are determined by the conversion of Cr(VI) in water with the presence of ethanol

The main content of the paper is divided into three parts Part I discusses the remark of water pollution and its impact to the human life, the wastewater treatment methods, review of TiO2 and precursors, the researches about TiO2 as a photocatalyst for wastewater treatment in the world and Viet Nam Part II introduces the synthesized catalytic methods, the methods to determine the composition of plating chromium wastewater, the methods to investigate the properties of the synthesized catalysts, the methods to calculate the reaction results Part III mentions the results and discussions of the experiments and the research process

CHAPTER I: LITERATURE REVIEW I.1 General of wastewater

I.1.1 Water pollution

Water pollution is currently one of the serious types of pollution facing human beings all over the world The issue has drawn different scales of involvement and cooperation

A case in point is that in European Union, a high level of environmental protection and the improvement of the quality of the water environment must be integrated into the policies of the Union and ensured in accordance with the principle of sustainable development The paper will go into depth of the problem in the perspectives of definition and causes of water pollution, and measurement of pollution and control of water pollution

In European Charter for sustainable tourism in protected area published in 2000, pollution of water is defined as `the discharge by man, directly or indirectly, of substances or energy into the aquatic environment, the results of which are such as to cause hazards to human health, harm to living resources and to aquatic eco-systems, damage to amenities or interference with other legitimate uses of water

There are also a variety of determinations of water pollution by different organizations However, they share together some common points as follows: (i) water pollution is the contamination of water bodies such as lakes, rivers, oceans, and groundwater; (ii) all water

Photo I.1: Water polluted by garbage

Pollution affects organisms and plants that live in these water bodies and in almost all cases the effect is damaging either to individual species and populations but also to the

natural biological communities; (iii) it occurs when pollutants are discharged directly or indirectly into water bodies without adequate treatment to remove harmful constituents [46]

I.1.2 Water pollution categories

Surface water and groundwater have often been studied and managed as separate resources, although they are interrelated [40] Sources of surface water pollution are generally grouped into two categories based on their origin

a Point source pollution

Point source pollution refers to contaminants that enter a waterway through a discrete conveyance, such as a pipe or ditch Examples of sources in this category include discharges from a sewage treatment plant, a factory, or a city storm drain

b Non-point source pollution

Non-point source (NPS) pollution refers to diffuse contamination that does not originate from a single discrete source NPS pollution is often accumulative effect of small amounts of contaminants gathered from a large area A typical example is that the leaching out of nitrogen compounds from agricultural land which has been fertilized Nutrient runoff in storm water from "sheet flow" over an agricultural field or a forest are also cited as examples of NPS pollution

c Groundwater pollution

Interactions between groundwater and surface water are complex Consequently,

groundwater pollution, sometimes referred to as groundwater contamination, is not as

easily classified as surface water pollution By its very nature, groundwater aquifers are susceptible to contamination from sources that may not directly affect surface water

bodies, and the distinction of point vs nonpoint source may be irrelevant

Groundwater accounts for 97% amount of fresh water of the Earth However, the water source is contaminated seriously The pollution may derive from fixing dumping ground unsanitarily, waste water from industrial activities, using a huge amount of fertilizer and pesticide The most popular ground water pollution is caused by As

Analysis of groundwater contamination may focus on soil characteristics and hydrology,

as well as the nature of the contaminant itself

I.1.3 Causes of polluted water

The specific contaminants leading to pollution in water include a wide spectrum of chemicals, pathogens, and physical or sensory changes such as elevated temperature and discoloration While many of the chemicals and substances that are regulated may be naturally occurring (calcium, sodium, iron, manganese, etc.) the concentration is often the key in determining what is a natural component of water, and what is a contaminant Many of the chemical substances are toxic Pathogens can produce waterborne diseases

in either human or animal hosts Alteration of water's physical chemistry includes acidity (change in pH), electrical conductivity, temperature, and eutrophication Eutrophication

is the fertilization of surface water by nutrients that were previously scarce

I.1.4 Effects of polluted water

There are various effects of water pollution

• Spread of disease: Drinking polluted water can cause cholera or typhoid infections, along with diarrhea

• Affects aody organs: The consumption of highly contaminated water can cause injury

to the heart and kidneys

• Harms the food chain: Toxins within water can harm aquatic organisms, thus breaking a link in the food chain

• Causes algae in water: Urea, animal manure and vegetable peelings are food for algae Algae grow according to how much waste is in a water source Bacteria feed off the algae, decreasing the amount of oxygen in the water The decreased oxygen causes harm to other organisms living in the water

• Flooding: The erosion of soil into waterways causes flooding, especially with heavy rainfall

• Harms animals: Birds that get into oil-contaminated water die from exposure to cold water and air due to feather damage Other animals are affected when they eat dead fish

in contaminated streams

• The effects of water pollution are not always immediate They are not always seen at the point of contamination They are sometimes never known by the person responsible for the pollution However, water pollution has a huge impact on our lives

I.1.5 Control of water pollution

a Domestic sewage

In urban areas, domestic sewage is typically treated by centralized sewage treatment plants Cities with sanitary sewer overflows or combined sewer overflows employ one or more engineering approaches to reduce discharges of untreated sewage, including:

• Utilizing a green infrastructure approach to improve stormwater management capacity throughout the system [42]

• Repair and replacement of leaking and malfunctioning equipment [43]

• Increasing overall hydraulic capacity of the sewage collection system (often a very

expensive option)

b Industrial wastewater

Some industrial facilities generate ordinary domestic sewage that can be treated by municipal facilities Industries that generate wastewater with high concentrations of conventional pollutants (e.g oil and grease), toxic pollutants (e.g heavy metals, volatile organic compounds) or other nonconventional pollutants such as ammonia, need specialized treatment systems Some of these facilities can install a pre-treatment system

to remove the toxic components, and then send the partially-treated wastewater to the municipal system Industries generating large volumes of wastewater typically operate their own complete on-site treatment systems

Some industries have been successful at redesigning their manufacturing processes to

reduce or eliminate pollutants, through a process called pollution prevention

Heated water generated by power plants or manufacturing plants may be controlled with:

• Cooling ponds, man-made bodies of water designed for cooling by evaporation, convection, and radiation

• Cooling towers, which transfer waste heat to the atmosphere through evaporation and/or heat transfer

• Cogeneration, a process where waste heat is recycled for domestic and/or industrial

heating purpose

c Agricultural wastewater

Nutrients (nitrogen and phosphorus) are typically applied to farmland as commercial fertilizer; animal manure; or spraying of municipal or industrial wastewater (effluent) or sludge Nutrients may also enter runoff from crop residues, irrigation water, wildlife, and atmospheric deposition Farmers can develop and implement nutrient management plans

to reduce excess application of nutrients

Pollution prevention practices include low impact development techniques, installation of green roofs and improved chemical handling (e.g management of motor fuels & oil, fertilizers and pesticides) [44] Runoff mitigation systems include infiltration basins, bioretention systems, constructed wetlands, retention basins and similar devices

d Thermal pollution

Thermal pollution from runoff can be controlled by stormwater management facilities that absorb the runoff or direct it into groundwater, such as bioretention systems and infiltration basins Retention basins tend to be less effective at reducing temperature, as the water may be heated by the sun before being discharged to a receiving stream

I.2 Cr(VI) treatment methods in wastewater

There are some methods to treat Cr(VI) in wastewater in the world

I.2.1 Ion exchange method

Whenever an ion is removed out of an aqueous solution and is replaced by another ionic species, this is what we generally refer to as “ion exchange” There are synthetic materials available that have been specially designed to enable ion exchange operations at high performance levels Among many other applications, these so called “ion exchangers” can be used in processes of environmental protection such as purification, decontamination, recycling or even for the design of new environment-friendly production processes Synthetic and industrially produced ion exchange resins consist of small, porous beads that are insoluble in water and organic solvents The most widely used base-materials are polystyrene and polyacrylate The diameter of the beads is in a range of 0.3 to 1.3 mm The beads contain around 50% of water, which is dispersed in the gel-structured compartments of the material Since water is dispersed homogenously through the bead, water soluble materials can move freely, in and out

To each of the monomer units of the polymer, so called “functional groups” are attached These functional groups can interact with water soluble species, especially with ions Ions are either positively (cat ions) or negatively (anions) charged Since the functional groups are also charged, the interaction between ions and functional groups is exhibited via electrostatic forces Positively charged functional groups (e.g a quarternary amine) interact with anions and negatively charged functional group (e.g a sulfonic, phosphonic

or carboxylic acid group) will interact with cations The binding force between the functional group and the attached ion is relatively loose The exchange can be reversed

by another ion passing across the functional group

Then another exchange reaction can take place and so on and so forth One exchange reaction can follow another [45]

I.2.2 Electrochemistry method

Principles: The method based on the redox processes to remove metals doped onto the electrodes from wastewater when a direct current beam went through the electrode

The unsoluble anode was made by graphite or lead oxide, the cathode was made by tungsten, iron or nickel Metallic ions were reduced at cathode to less poisonous forms or metal

Mem+ + (m-n)e+ Men+ (m>n≥0)

(m,n: oxidation number of metal Me)

I.2.3 Reduction-oxidation method and deposition method

I.2.4 Photocatalytic method

All of the extensive knowledge that was gained during the development of semiconductor photoelectrochemistry during the 1970 and 1980s has greatly assisted the development of photocatalysis In particular, it turned out that TiO2 is excellent for photocatalytically breaking down organic compounds For example, if one puts catalytically active TiO2

powder into a shallow pool of polluted water and allows it to be illuminated with sunlight, the water will gradually become purified Ever since 1977, when Frank and Bard first examined the possibilities of using TiO2 to decompose cyanide in water, there has been increasing interest in environmental applications These authors quite correctly pointed out the implications of their result for the field of environmental purification Their prediction has indeed been borne out, as evidenced by the extensive global efforts

in this area.One of the most important aspects of environmental photocatalysis is the availability of a material such as titanium dioxide, which is close to being an ideal photocatalyst in several respects For example, it is relatively inexpensive, highly stable

chemically, and the photogenerated holes are highly oxidizing In addition, photogenerated electrons are reducing enough to produce superoxide from dioxygen

I.3 TiO 2 review

I.3.1 Occurrence

Titanium dioxide occurs in nature as well-known minerals rutile, anatase and brookite, and additionally as two high pressure forms, a monoclinic baddeleyite-like form and an orthorhombic α-PbO2-like form, both found recently at the Ries crater in Bavaria The most common form is rutile, which is also the most stable form Anatase and brookite both convert to rutile upon heating Rutile, anatase and brookite all contain six coordinated titanium.[48]

I.3.2 Physical and mechanical properties

Physical and mechanical properties of sintered titania are summarised in table 1, while optical properties of titania are provided in table 2

Table I.1 Typical physical and mechanical properties of titania [47]

Property

Fracture Toughness 3.2 Mpa.m-1/2

I.3.3 Chemical properties

TiO2 is nonreactive substance, did not react with water, did not disolve in low acid solution (except HF) and base solution

a Pigments

The most important function of titanium dioxide however is in powder form as a pigment for providing whiteness and opacity to such products such as paints and coatings (including glazes and enamels), plastics, paper, inks, fibres and food and cosmetics

Titanium dioxide is by far the most widely used white pigment Titania is very white and has a very high refractive index – surpassed only by diamond The refractive index determines the opacity that the material confers to the matrix in which the pigment is housed Hence, with its high refractive index, relatively low levels of titania pigment are required to achieve a white opaque coating

The high refractive index and bright white colour of titanium dioxide make it an effective opacifier for pigments The material is used as an opacifier in glass and porcelain enamels, cosmetics, sunscreens, paper, and paints One of the major advantages of the material for exposed applications is its resistance to discoloration under UV light [47]

b Photocatalysis

Titanium dioxide, particularly in the anatase form, is a photocatalyst under ultraviolet (UV) light Recently it has been found that titanium dioxide, when spiked with nitrogen ions or doped with metal oxide like tungsten trioxide, is also a photocatalyst under either

visible or UV light The strong oxidative potential of the positive holes oxidizes water to create hydroxyl radicals It can also oxidize oxygen or organic materials directly Titanium dioxide is thus added to paints, cements, windows, tiles, or other products for its sterilizing, deodorizing and anti-fouling properties and is used as a hydrolysis catalyst

It is also used in dye-sensitized solar cells, which are a type of chemical solar cell (also known as a Graetzel cell) The photocatalytic properties of titanium dioxide were discovered by Akira Fujishima in 1967 and published in 1972 The process on the surface

of the titanium dioxide was called the Honda-Fujishima effect Titanium dioxide has potential for use in energy production as a photocatalyst.[48]

c Oxygen Sensors

Even in mildly reducing atmospheres titania tends to lose oxygen and become sub stoichiometric In this form the material becomes a semiconductor and the electrical resistivity of the material can be correlated to the oxygen content of the atmosphere to which it is exposed Hence titania can be used to sense the amount of oxygen (or reducing species) present in an atmosphere [47]

d Antimicrobial Coatings

The photocatalytic activity of titania results in thin coatings of the material exhibiting self cleaning and disinfecting properties under exposure to UV radiation These properties make the material a candidate for applications such as medical devices, food preparation surfaces, air conditioning filters, and sanitaryware surfaces [47]

I.4 Mechanism of titanium oxide photocatalytic reactions

I.4.1 Band structure of semiconductors and band gap energy

If the nucleus of an atom were the sun in our solar system, the electrons revolving around the nucleus would be the orbiting planets The path that an electron travels is referred to as an "orbit." There is a limit to the number of electrons that can occupy one orbit Electrons in the outermost orbit are referred to as "valence electrons." Valence electrons are responsible for the bonding of atoms When there are few atoms, the energy values of electrons in orbits are scattered However, when the number of bonded atoms increases, the values become continuous within a certain range, rather than being scattered This range is referred to as an "energy band." The area between two energy bands, where there is no electron energy, is referred to as a "forbidden band." Among the bands filled with electrons, the one with the highest energy level (the electron orbit farthest from the nucleus) is referred to as the "valence band," and the band outside of this is referred to as the "conduction band." The energy width of the forbidden band between the valence band and the conduction band is referred to as the "band gap."

The band gap is like a wall that electrons must jump over in order to become free The amount of energy required to jump over the wall is referred to as the "band-gap energy." Only electrons that jump over the wall and enter the conduction band (which are referred

to as "conduction electrons") can move around freely In the case of silicon, the band gap energy is approximately 1.1 eV, which is equal to approximately 1100 nm when converted to the wavelength of light When rutile type titanium oxide and anatase type titanium oxide are irradiated with light of 413 nm or lower, or 388 nm or lower, respectively, valence band electrons move up to the conduction band At the same time,

as many positive holes as the number of electrons that have jumped to the conduction band are created [49]

I.4.2 Energy structure of titanium oxide and photoeffect

In a compound semiconductor consisting of different atoms, the valence band and conduction band formation processes are complicated, but the principles involved are the same For example, it is known that the valence band of titanium oxide is comprised of the 2p orbital of oxygen (O), while the conduction band is made up of the 3d orbital of titanium (Ti) In a semiconductor with a large band gap, electrons in the valence band cannot jump up to the conduction band However, if energy is applied externally, electrons in the valence band can rise (this is referred to as "excitation") to the conduction band Consequently, as many electron holes (holes left behind by the electrons moving up

to the conduction band) as the number of excited electrons are created in the valence band This is equivalent to the movement of electrons from the bonding orbital to the antibonding orbital In other words, the photoexcited state of a semiconductor is generally unstable and can easily break down

Titanium oxide, on the other hand, remains stable even when it is photoexcited This is one of the reasons that titanium oxide makes an excellent photocatalyst The following three factors pertaining to the band structure of semiconductors have the greatest effect

on photocatalytic reactions:

(1) Band gap energy

(2) Position of the lowest point in the conduction band

(3) Position of the highest point in the valence band

In photocatalytic reactions, the band gap energy principally determines which light wavelength is most effective, and the position of the highest point in the valence band is the main determinant of oxidative decomposing power of photocatalyst [49]

Fig.I.1 Titanium-oxide band structure [49]

I.4.2 Crystal structures and photocatalytic activity of titanium oxide

There are three types of crystal structures in natural titanium oxide: the rutile type, the anatase type, and the brookite type All three of these types are expressed using the same chemical formula (TiO2); however, their crystal structures are different Titanium oxide absorbs light having an energy level higher than that of the band gap, and causes electrons to jump to the conduction band to create positive holes in the valence band Despite the fact that the band gap value is 3.0 eV for the rutile type and 3.2 eV for the anatase type, they both absorb only ultraviolet rays However, the rutile type can absorb the rays that are slightly closer to visible light rays

As the rutile type can absorb light of a wider range, it seems logical to assume that the rutile type is more suitable for use as a photocatalyst However, in reality, the anatase type exhibits higher photocatalytic activity One of the reasons for this is the difference in the energy structure between the two types In both types, the position of the valence band is deep, and the resulting positive holes show sufficient oxidative power However, the conduction band is positioned near the oxidation-reduction potential of the hydrogen, indicating that both types are relatively weak in terms of reducing power It is known that the conduction band in the anatase type is closer to the negative position than in the rutile type; therefore, the reducing power of the anatase type is stronger than that of the rutile type Due to the difference in the position of the conduction band, the anatase type exhibits higher overall photocatalytic activity than the rutile type [49]

Fig I.2 Crystal structures of titanium oxide [49]

I.4.3 Effect of ultraviolet rays in activating titanium oxide

The band gap of anatase type titanium oxide is 3.2 eV, which is equivalent to a wavelength of 388 nm The absorption of ultraviolet rays shorter than this wavelength promotes reactions These ultraviolet rays are near-ultraviolet rays contained in the sunlight reaching the earth and emitted by room lights, and they have a very limited range

of weak light throughout the spectrums of sunlight and room lights

The development of a visible-light photocatalyst may be considered as a solution, but no substance superior to titanium oxide as a material for photocatalysts has yet been discovered One major reason for this is that a semiconductor with a smaller band gap than that of titanium oxide results in autolysis if it receives light in the presence of water

In titanium oxide, the absorption of ultraviolet rays with a wavelength of 388 nm or shorter promotes reactions; however, it is known that 254-nm rays having a greater energy level, which are used in germicidal lamps, are absorbed by the DNA of living organisms and form pyrimidine dimers, thereby damaging the DNA

Titanium oxide photocatalyst does not require ultraviolet rays that have an energy level

as high as 254 nm and are hazardous to humans It also allows reactions to be initiated by the near-ultraviolet rays with relatively long wavelengths contained in sunlight and emitted by fluorescent lamps [49]

Table I.3 Ultraviolet rays in ordinary surroundings [49]

I.4.4 Decomposing power of titanium oxide photocatalyst

When light is absorbed by titanium oxide, two carrier electrons (e-) and positive holes (h+) are formed In ordinary substances, electrons and positive holes recombine quickly; however, in titanium oxide photocatalyst they recombine more slowly The percentage of carrier recombination has a major effect on the photocatalytic efficiency

Fig.I.3 Electron structure of titanium oxide [49]

One of the notable features of titanium oxide is the strong oxidative decomposing power

of positive holes, which is greater than the reducing power of electrons excited to the conduction band The surface of a photocatalyst contains water, which is referred to as

"absorbed water." When this water is oxidized by positive holes, hydroxy radicals (• OH),

which have strong oxidative decomposing power, are formed Then, the hydroxy radicals react with organic matter If oxygen is present when this process takes place, the intermediate radicals in the organic compounds and oxygen molecules can undergo radical chain reactions and consume oxygen in some cases In such a case, the organic matter eventually decomposes, ultimately becoming carbon dioxide and water Under some conditions, organic compounds can react directly with the positive holes, resulting

in oxidative decomposition Meanwhile, the reduction of oxygen contained in the air occurs as a pairing reaction As oxygen is an easily reducible substance, if oxygen is present, the reduction of oxygen takes place instead of hydrogen generation The reduction of oxygen results in the generation of superoxide anions (• O2-) Superoxide anions attach to the intermediate product in the oxidative reaction, forming peroxide or changing to hydrogen peroxide and then to water

Fig I.4 Oxidation mechanism[49]

Fig.I.5 Reduction mechanism[49]

As reduction tends to occur more easily in organic matter than in water, when the concentration of organic matter becomes high, the possibility of positive holes being used

in the oxidative reactions with organic matter increases, thus reducing the rate of carrier recombination It is believed that, under conditions in which positive holes are

sufficiently consumed, the process of electrons transferring to oxygen molecules on the reduction side determines the reaction speed of the entire photocatalytic reaction In other words, by enabling easier transfer of electrons to oxygen molecules, the efficiency of photocatalytic reactions can be improved This can be achieved by allowing titanium oxide to carry a metal as a support.[49]

1.4.5 The mechanism of photo-reduction of Cr(VI)

The photo-reduction of Cr(VI) toCr(III) can be achieved via a photocatalytic process with

a simplified mechanism as follows:

UV light illumination on TiO2 produces hole–electron pairs (reaction (1)) at the surface

of the photocatalyst After the hole–electron pairs being separated, the electrons can reduce Cr(VI) to Cr(III) (reaction (2)), and the holes may lead to generation of O2 in the absence of any organics (reaction (3))

Therefore, in a completely inorganic aqueous solution, the net photocatalytic reaction is the three-electron-reduction of Cr(VI) to Cr(III) with oxidation of water to oxygen, which

is a kinetically slow four-electron process And hence the photocatalytic reduction of Cr(VI) alone is quite slow Alternatively, the photocatalytic reduction of Cr(VI) can be carried out in couple with the photocatalytic oxidation of organic pollutants by adding some amount of organic pollutants in solution In the presence of degradable organic pollutants, the holes can produce •OHradicals (reaction (4)), which can further degrade the organics to CO2 and H2O (reaction (5)) Of course, the holes can also directly oxidize the organic molecules (reaction (6)) In otherPositive holes (h+) that cause oxidative reaction have very strong oxidative power They directly oxidize water and produce a highly reactive compound [OH] In some cases, they directly oxidize organic matter attached to the surface.Radical chain reactions also occur between the radicals and the oxygen molecules (reaction (6)) In other words, in the presence of organic species, the photogenerated holes are rapidly scavenged from the TiO2 particles, suppressing electron–hole recombination on TiO2 and accelerating the reduction of Cr(VI) by photogenerated electron One of the important strategies of promoting the photocatalytic

reduction of Cr(VI) (and the photocatalytic degradation of organic pollutants) is

enhancing the charge separation, which can be achieved by improving the structure of the

photocatalyst and by introducing scavengers of holes and/or electrons in the solution.[60]

I.5 Review of the used precursors in this thesis

I.5.1 TiCl 4

Titanium tetrachloride is the inorganic compound with the formula TiCl4 It is an

important intermediate in the production of titanium metal and the pigment titanium

dioxide TiCl4 is an unusual example of a metal halide that is highly volatile Upon

contact with humid air, it forms spectacular opaque clouds of titanium dioxide (TiO2) and

hydrogen chloride (HCl)

a Properties and structure

Table I.4 The physical properties of TiCl 4

Physical Properties

TiCl4 is a dense, colourless distillable liquid, although crude samples may be yellow or

even red-brown It is one of the rare transition metal halides that is a liquid at room

temperature, VCl4 being another example This property reflects the fact that TiCl4 is

molecular; that is, each TiCl4 molecule is relatively weakly associated with its

neighbours Most metal chlorides are polymers, wherein the chloride atoms bridge

between the metals The attraction between the individual TiCl4 molecules is weak,

primarily van der Waals forces, and these weak interactions result in low melting and boiling points, similar to those of CCl4

Ti4+ has a "closed" electronic shell, with the same number of electrons as the inert gas argon The tetrahedral structure for TiCl4 is consistent with its description as a d0 metal center (Ti4+) surrounded by four identical ligands This configuration leads to highly symmetrical structures, hence the tetrahedral shape of the molecule TiCl4 adopts similar structures to TiBr4 and TiI4; the three compounds share many similarities TiCl4 and TiBr4 react to give mixed halides TiCl4-xBrx, where x = 0, 1, 2, 3, 4 Magnetic resonance measurements also indicate that halide exchange is also rapid between TiCl4 and VCl4 TiCl4 is soluble in toluene and chlorocarbons, as are other non-polar species Evidence exists that certain arenes form complexes of the type [(C6R6)TiCl3]+ TiCl4 reacts exothermically with donor solvents such as THF to give hexacoordinated adducts Bulkier ligands (L) give pentacoordinated adducts TiCl4L.[46]

b Production

TiCl4 is produced by the chloride process, which involves the reduction of titanium oxide ores, typically ilmenite (FeTiO3) with carbon under flowing chlorine at 900 °C Impurities are removed by distillation

2 FeTiO3 + 7 Cl2 + 6 C → 2 TiCl4 + 2 FeCl3 + 6 CO

The coproduction of FeCl3 is undesirable, which has motivated the development of alternative technologies Instead of directly using ilmenite, "rutile slag" is used This material, an impure form of TiO2, is derived from ilmenite by removal of iron, either using carbon reduction or extraction with sulfuric acid Crude TiCl4 contains a variety of other volatile halides, including vanadyl chloride (VOCl3), silicon tetrachloride (SiCl4), and tin tetrachloride (SnCl4), which must be separated.[46]

c Applications

• Production of titanium metal

The world's supply of titanium metal, about 4M tons per year, is made from TiCl4 The conversion takes place by the reduction of the chloride with magnesium metal, and yields titanium metal and magnesium chloride This procedure is the final step of the Kroll process:

2 Mg + TiCl4 → 2 MgCl2 + Ti

Liquid sodium has also been used instead of magnesium as the reducing agent

• Production of titanium dioxide

Around 90% of the TiCl4 production is used to make the pigment titanium dioxide (TiO2) The conversion involves hydrolysis of TiCl4, a process that forms hydrogen chloride:

• Hydrolysis and related reactions

The most noteworthy reaction of TiCl4 is its easy hydrolysis, signaled by the release of corrosive hydrogen chloride and the formation of titanium oxides and oxychlorides, as described above for the production of TiO2 In the past titanium tetrachloride has also been used to create naval smokescreens The hydrogen chloride immediately absorbs more water to form tiny droplets of hydrochloric acid, which (depending on humidity) may absorb still more water, to produce large droplets that efficiently scatter light In addition, the highly refractive titanium dioxide is also an efficient light scatterer Because

of the corrosiveness of its smoke, however, TiCl4 is no longer used

Alcohols react with TiCl4 to give the corresponding alkoxides with the formula [Ti(OR)4]n (R = alkyl, n = 1, 2, 4) As indicated by their formula, these alkoxides can adopt complex structures ranging from monomers to tetramers Such compounds are useful in materials science as well as organic synthesis A well known derivative is titanium isopropoxide, which is a monomer

Organic amines react with TiCl4 to give complexes containing amido (R2N--containing) and imido (RN2--containing) complexes With ammonia, titanium nitride is formed An illustrative reaction is the synthesis of tetrakis(dimethylamido)titanium Ti(NMe2)4, a yellow, benzene-soluble liquid: This molecule is tetrahedral, with planar nitrogen centers

4 LiNMe2 + TiCl4 → 4 LiCl + Ti(NMe2)4

• Complexes with simple ligands

TiCl4 is a Lewis acid as implicated by its tendency to hydrolyze With the ether THF, TiCl4 reacts to give yellow crystals of TiCl4(THF)2 With chloride salts, TiCl4 reacts to form sequentially [Ti2Cl9]−, [Ti2Cl10]2− (see figure above), and [TiCl6]2− Interestingly, the reaction of chloride ions with TiCl4 depends on the counterion NBu4Cl and TiCl4

gives the pentacoordinate complex NBu4TiCl5, whereas smaller NEt4+ gives (NEt4)2Ti2Cl10 These reactions highlight the influence of electrostatic forces on the structures of compounds with highly ionic bonding

• Redox

Reduction of TiCl4 with aluminium results in one-electron reduction The trichloride

(TiCl3) and tetrachloride have contrasting properties: the trichloride is a solid, being a coordination polymer, and is paramagnetic When the reduction is conducted in THF solution, the Ti(III) product converts to the light-blue adduct TiCl3(thf)3

• Organometallic chemistry

The organometallic chemistry of titanium typically starts from TiCl4 An important reaction involves sodium cyclopentadienyl to give titanocene dichloride, TiCl2(C5H5)2 This compound and many of its derivatives are precursors to Ziegler-Natta catalysts Tebbe's reagent, useful in organic chemistry, is an aluminium-containing derivative of titanocene that arises from the reaction of titanocene dichloride with trimethylaluminium

It is used for the "olefination" reactions

Arenes, such as C6(CH3)6 react to give the piano-stool complexes [Ti(C6R6)Cl3]+ (R = H,

CH3; see figure above) This reaction illustrates the high Lewis acidity of the TiCl3+

entity, which is generated by abstraction of chloride from TiCl4 by AlCl3

• Reagent in organic synthesis

TiCl4 finds limited but diverse use in organic synthesis, capitalizing on its Lewis acidity and its oxophilicity.[8] Illustrative is the Mukaiyama aldol reaction Key to this application is the tendency of TiCl4 to activate aldehydes (RCHO) by formation of adducts such (RCHO)TiCl4OC(H)R It is also used in the McMurry reaction in conjunction with zinc, LiAlH4 These reducing agents generate Ti(III) derivatives that couple ketones, leading to alkenes

• Toxicity and safety considerations

Hazards posed by titanium tetrachloride generally arise from the release of hydrogen chloride (HCl) TiCl4 is a strong Lewis acid, exothermically forming adducts with even weak bases such as THF and explosively with water, releasing HCl.[48]

I.5.2 Titanium isopropoxide (TTIP)

Titanium isopropoxide is a chemical compound with the formula Ti{OCH(CH3)2}4 This alkoxide of titanium(IV) is used in organic synthesis and materials science

The structures of the titanium alkoxides are often complex Crystalline titanium methoxide is tetrameric with the molecular formula Ti4(OCH3)16 Alkoxides derived from bulkier alcohols such isopropanol aggregate less Titanium isoproxide is mainly a monomer in nonpolar solvents It is a diamagnetic tetrahedral molecule

a Properties

Table I.5.The physical properties of TTIP

Physical Properties Molecular formula C12H28O4Ti

Solubility in water Reacts to form TiO2,

Solubility soluble in many organic solvents

Titanium isopropoxide reacts with water to deposit titanium dioxide at ambient temperature:

Ti{OCH(CH3)2}4 + 2 H2O → TiO2 + 4 (CH3)2CHOH

This reaction is employed in the sol-gel synthesis of TiO2-based materials Typically water is added to a solution of the alkoxide in an alcohol The nature of the inorganic product is determined by the presence of additives (e.g acetic acid), the amount of water, and the rate of mixing

Titanium isopropoxide is a component of the Sharpless epoxidation, a method for the synthesis of chiral epoxides The compound is also used as a catalyst for the preparation

of certain cyclopropanes in the Kulinkovich reaction Prochiral thioethers are oxidized enantioselectively using a catalyst derived from Ti(O-i-Pr)4.[46]

I 6 Literature review about using TiO 2 as a photocatalyst for wastewater treatment

TiO2 was widely used as catalyst for many water and air treatments In water treatments, Titannium oxide was known as photocatalyst In Viet Nam and over the word, there are a lot of researches about the degradation of organic species and the reduction of heavy metals from water using TiO2 as catalyst and ultraviolet(UV) light

I.6.1 In Vietnam

The surface of titannium oxide by dispersing was modified many oxides onto the surface

to form a monolayer When the catalysts are up to the monolayer, the oxide remains on the support as amorphous phase or in other words, it is well dispersed Above the monolayer, X-ray diffractrograms indicate the presence of crystalline oxide The monolayer of MeOx/TiO2 catalysts (Me=Mo, Co, Ni) [1] P25 Degussa was used as a catalyst for the degradation of reactive blue 2(RB2) under UV-light The organic part was totally converted into innocuous diluted inorganic final products such as sulfate and nitrate ions The carboxylic acids and intermediates were analysed by high performance liquid chromatography (HPLC) Six carboxylic acids, oxalic acid (Rt = 6.23 min), tactaric acid (Rt = 7.9 min), maleic acid (Rt = 8.3 min), formic acid (Rt = 12.51 min), acetic acid (Rt = 14.12 min), fumaric (Rt = 16.01 min), have been found Ammeline, ammelide and cyanuric acid were determined The pathway of the ammeline-ring degradation has been established [2]

Nguyen Van Dung et al [3] showed that nano size powder of TiO2 powder was synthesized from Viet Nam pure ilmenite material by hydrolysis processes with microwave condition Average size of anatase phase is about 9-40nm In this paper, photocatalyst TiO2 activity was evaluated by the degradation of reactive orange 10 acid The result showed that there was the relation between crystallizable anatase phase and photocatalytic activity The best photocatalytic activity was reached by crystallizable anatase sample calcinated at transition phase temperature with average size of anatase phase about 20 nm

Ngo Tuan Anh et al [4] studied “Composite TiO2/Carbon nano” based photocatalysts because of their high photoactivity and their capacity of absorbing almost of sun’s irradiation By absorbing a photon, these catalysts can promote the total oxidation of organic compounds to CO2, H2O products The research focused on the new photocatalysts based on “composite TiO2 commercial, sol-gel / Carbon nano” with macroscopic structure This new type of catalyst allows amplifying the photoactivity and

reducing the cost These catalysts were successfully applied in our laboratory for continuous systems to degradate organic compounds in waste water

I.6.2 In the world

There are many methods to do research about TiO2 as a photocatalyst in the word The differences among these studies are treated heavy metals and organic species

a Using only TiO 2 as a photocatalyst

TiO2 was investigated as a singular photocatalyst TiO2 can be use many form such as powder

In the research of S Watson, Nanocrystalline titanium dioxide (TiO2) particles were prepared by a modified alkoxide method under acidic conditions at temperatures ranging from 60◦C to 90◦C The photocatalytic activities of the prepared nanosized TiO2 were compared to those obtained from Degussa P-25 TiO2 as well as TiO2 crystalline samples prepared using the conventional sol–gel/heat treatment method At low organic concentrations, Degussa P-25 exhibited higher photocatalytic behaviour than all the prepared particles while, at high organic concentrations, the nanosized TiO2 particles prepared at low temperature displayed an activity comparable to Degussa P25 but much higher than the heat treated sample [5]

The photocatalytic reduction of Cr(VI) over TiO2 catalysts was investigated in a large number of research with both the absence and presence of organic compounds [6, 7,8, 9,…] The photocatalytic reduction of Cr(VI) behaved as a pseudo-first-order reaction in kinetics Treatment efficiency for organics/TiO2 system generally increased with increasing catalyst loading, decreasing solution pH and it was also promoted in the presence of dissolved oxygen In the absence of any organic species, the rate constant

(kCr) for the photocatalytic reduction of Cr(VI) was found to be increased initially,

passing a maximum, and then decreased, as calcination temperature was increased In the

presence of organic compounds, however, kCr was decreased with the increase of

calcination temperature.During the photocatalytic reaction, the total removal of Cr(VI) occurred through adsorption onto TiO2, as well as its reduction to Cr(III) These results demonstrated that the photocatalytic reduction of Cr(VI) alone was dependent on both of specific surface area and crystalline structure of the photocatalyst in the absence of any organic compounds, but was dominated by the specific surface area of the photocatalyst

in the presence of organic compounds because of the synergistic effect between the photocatalytic reduction of Cr(IV) and the photocatalytic oxidation of organic compounds

Photoreduction/removal of cadmium was studied at some researches Vi Nu Hoai Nguyen

et al [10] Photoreduction/removal of cadmium was studied at pH 7 using TiO2 Degussa

as photocatalyst, and either formate or methanol as hole scavengers In the absence of organic additives, approximately 60% of 30 ppm cadmium was found to be removed from the solution by adsorption, however no cadmium reduction was observed when methanol was added as the hole scavenger It was found that the adsorption of both cadmium and the organic hole scavenger is crucial for the photoreduction of cadmium Andrea Cavicchioli et al [11] investigated the influence of electron and hole scavengers

in the photocatalytic digestion of organic matter in the presence of suspended particles of TiO2 The process, aiming at the electrochemical determination of traces of heavy metals

in water samples, was followed through the recovery of the voltammetric wave of Cd(II)

in the presence of EDTA The accelerating power of O2, acting as electron scavenger, was confirmed and CH3OH exhibits an antagonist effect as hole scavenger Furthermore, the effect of pH, initial concentrations of formic acid and selenate (Se(VI)) ions on the UV/TiO2 reduction of Se(VI) ions was investigated by Timothy T.Y Tan [12] The optimum molar adsorption ratio could be achieved by manipulating the initial pH, initial solute concentration and the order of which the solutes were adsorbed

TiO2 was also used as a photocatalyst for oxidation of organic compounds Mari Iizuka et

al [13, 14, 15] investigated photoirradiation of titanium oxide (TiO2) excites the electrons from the valence band to the conduction band, leaving holes in the valence band The photocatalytic perfluoroalkylation of aromatic rings such as benzene and its derivatives, naphthalene and benzofuran with perfluoroalkyl iodide by the combination of reduction and oxidation reactions with TiO2 was studied The results showed that perfluoroalkyl iodide was reduced to a perfluoroalkyl radical by the excited electrons in the conduction band of TiO2, and the resulting radical reacted with an aromatic ring to form an arenium radical that was successively oxidized to a cation by the holes in the valence band of TiO2

b Using TiO 2 loading onto supports

TiO2 was doped onto various supports to synthesize large specific surface area (SSA) TiO2 photocatalyst loaded onto activated carbon (AC) support have been developed over the last decade [17] This photocatalyst has been used in a variety of investigations, i.e from water decontamination to direct pollutant degradation in aqueous and gas phase systems using UV irradiation and lately with the assistance of ultrasonic sound waves The highly dispersed titanium oxide catalysts have been prepared within zeolite cavities

as well as in the zeolite framework and utilized as photocatalysts [18, 19] However, the highest SSA of the synthesized catalysts was TiO2/MCM-41system [55-59] The SSA of

TiO2/MCM-41 systems can reach 800m2/g, and their photocatalyst activies very high Glass plates were coated with TiO2 in a photocatalytic process to collect mercury, lead, copperand, cadmium from aqueous solutions containing individual metals and mixtures [20] It was demonstrated that 100 mL solutions containing 10 ppm of each of the four metals could be treated with a 10 cm2 TiO2-coated plate to leave undetectable metal concentrations in one hour Data were also obtained to show the effectiveness in treating silver containing solutions, indicating suitability of the photocatalytic process in treating photographic processing wastes

S Kment reported on preparation of titanium (IV) oxide thin films by a series of chemical, physico-chemical and physical methods including the sol–gel process carried out in the environment of lyotropic liquid crystals, Barrier-torch Discharge deposition, Magnetron Sputtering and the Modulated Hollow Cathode Plasma Jet Sputtering The produced layers have been thoroughly described by means of a series of characterization techniques including atomic force microscopy, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, ultravioletvisible spectroscopy, Fourier transformed infrared spectroscopy, Raman spectroscopy, ellipsometry, profilometry and surface wettability The films were then used as photoactive species in catalytic oxidation tests based on photoinduced decomposition of methylester of stearic acid [16]

c Using other elements doped TiO 2

Some authors use other elments such as Fe, Cu, N, Ni, Pd… doped titanium oxide to synthesise new photocatalyst

Nitrogen-doped titanium oxide (TiOxNy) as a photocatalyst was prepared by various methods such as a sol–gel process, wet process in some researches [21, 22, 23, 62] The nitrogen is readily doped in TiO2 by energetic nitrogen ions in the plasma and the films exhibited photocatalytic properties under visible light The roles N was discussed in terms of the production and separation of the charge carriers under visible-light illumination The photocatalytic activity was thus dependent on dopants and light intensity

S-doped TiO2 photocatalyst with high visible light activity was prepared by acid catalyzed hydrolysis method using thiourea (TU) as sulfur source [30, 31, 63] It was found that cation S6+ was homogeneously incorporated into the bulk phase of TiO2 and substitutes for some of the lattice titanium (Ti4+) Doped S can form a new band above the valence band and narrow the band-gap of the photocatalyst, giving rise to a second absorption edge in the visible light region The activity of the catalyst was examined by photodegradation of phenol in aqueous solution under both artificial visible light and

solar light irradiation Catalyst with optimum S-doping exhibited the highest activity under both artificial light and solar irradiation for phenol degradation In addition, titania doped S is also beneficial for the better dispersion, large BET and phase transformation retardation of TiO2 The high activity of S-doped TiO2 (STO) can be related to the results

of the synergetic effects of strong absorption in the UV–vis region, red shift in adsorption edge, oxygen vacancies and the enhancement of surface acidity induced by S codoping The nanoparticles of TiO2 modified with carbon and iron were synthesized by sol–gel followed solvothermal method at low temperature [60, 61] It was found that carbon and iron modification causes the absorption edge of TiO2 to shift the visible light region Fe(III) cation could be doped into the matrix of TiO2, by which could hinder the recombination rate of excited electrons/holes Superior photocatalytic activity of TiO2

modified with carbon and iron was observed for the decomposition of acid orange 7 (AO7) under visible light irradiation The synergistic effects of carbon and iron inmodified TiO2 nanoparticles were responsible for improving visible light photocatalytic activity

Titanium(IV) oxide (TiO2) particles were modified with several kinds of transition-metal ions (iron(III), copper(II), nickel(II) and chromium(III) ions) [32, 33, 34] Photocatalytic activity for acetaldehyde decomposition over metal-ion-modified TiO2, especially iron(III), showed higher photocatalytic activity than bare TiO2 under ultraviolet (UV) as well as visible-light irradiation Double-beam photoacoustic measurements suggested that metal ions on TiO2 surface acted differently depending on the wavelength of photoirradiation, i.e., as electron acceptors under UV irradiation and as electron injectors under visible-light irradiation

Junxi Liu et al [25] showed that a titanium-boron binary oxide has been prepared by gel method and used as a photocatalyst for the decomposition of water The structure of titanium oxide species in the Ti/B binary oxide was amorphous before and crystal after calcination in O2, while the boron oxide species maintained its amorphous state With increasing calcination temperature, the crystalline structure of titanium oxides changed from an anatase phase to a rutile phase Pt-loaded Ti/B photocatalysts decomposed water stoichiometrically in aqueous suspension system Their photocatalytic activity decreased markedly with increase in the calcination temperature, indicating that the photocatalytic activity of the Ti/B binary oxide was strongly dependent on the crystal phase of the titanium oxide in the Ti/B binary oxide A remarkable yield in the reaction of water decomposition was obtained when Na2CO3 was added in the Pt-loaded Ti/B binary oxide suspension

sol-Takeshi Kudo et al [26] investigated that the development of highly efficient TiO2

photocatalysts that can be directly anchored onto a substrate through chemical bonds such

as –Si–O–Ti– was carried out in order to develop an effective and stable air purification system The results obtained from the present study are as follows Highly active

‘‘rectangular column-structured TiO2 crystals’’ which could be anchored onto silica sheets were developed The rectangular column-structured TiO2 crystals could be anchored perpendicularly onto a silica fiber substrate in a very dense state with stable chemical bonds The TiO2 crystals had a width of 100–500 nm and length of 1000–5000

nm, consisting of anatase TiO2 nanoparticles of 10–30 nm Moreover, the rectangular columnar crystals were observed to have a hollow structure Investigations on the complete oxidation reaction of acetaldehyde into CO2 and H2O showed a high performance for such rectangular columnstructured TiO2 photocatalysts equivalent to or even higher than the most efficient standard P-25 powdered photocatalyst Effective and stable air purifying systems could be successfully developed by applying these TiO2

photocatalyst sheets for the complete oxidation of organic compounds and bacteria in the gas phase

Hiromi Yamashita et al [27] synthesized the F- media, the hydrophobic zeolite and mesoporous silica These hydrophobic porous materials exhibit the high ability for the adsorption of organic compounds diluted in water and become the useful supports of photocatalyst The hydrophobic Ti-Beta(F) zeolite prepared in the F- media exhibited high efficiency than the hydrophilic Ti-Beta(OH) zeolite prepared in OH- media for the liquid-phase photocatalytic degradation of 2-propanol diluted in water to produce CO2

and H2O The TiO2 loaded on the hydrophobic mesoporous silica HMS(F) (TiO2/HMS(F)), which was synthesized using tetraethyl orthosilicate, tetraethylammonium fluoride as the source of the fluoride and dodecylamine as templates, also exhibited the efficient photocatalytic performance for the degradation The amount of adsorption of 2-propanol and the photocatalytic reactivity for the degradation increased with increasing the content of fluoride ions on these photocatalysts The efficient photocatalytic degradation of 2-propanol diluted in water on Ti-Beta(F) zeolite and TiO2/HMS(F) mesoporous silica can be attributed to the larger affinity for the adsorption of propanol molecules on the titanium oxide species depending on the hydrophobic surface properties of these photocatalysts

Keiji Hashimoto et al [28] synthesised titanium(IV) oxide samples modified with platinum or rhodium chloride (H2PtCl6/TiO2 or RhCl3/TiO2) by an impregnation method and post-calcination at various temperatures and were used for photo-oxidation of

nitrogen oxide under irradiation of visible light or UV light Turnover numbers of both the catalysts were maintained at temperatures up to 3500C under 24-h irradiation of visible light, although the specific surface area of the catalysts decreased greatly with increase in post-calcination temperature The turnover number of H2PtCl6/TiO2 was about two-times larger than that of RhCl3/TiO2 Only a small amount of released NO2 was observed in the RhCl3/TiO2 catalyst, whereas in the H2PtCl6/TiO2 catalyst, the amount of

NO2 released to gas phase increased with an increase in oxidation products The small amount of released NO2 indicates that most of the NOx adsorbed on RhCl3/TiO2as an adsorption form of nonvolatile NO3-, whereas the amount of adsorbed NO2 on

H2PtCl6/TiO2 was about four-times larger than that on RhCl3/TiO2 The results indicate that the oxidation rate of NO2 to NO3- over RhCl3/TiO2was faster than that over

H2PtCl6/TiO2 These results strongly suggest that the Cl radical induced by visible light was not directly related to the photo-oxidation of NO to NO2 and NO3- and that the complex species of RhCl3 and H2PtCl6 contributed to the photo-oxidation

Victoria Apostolopoulou et al [29] showed [60] fullerene is a promising deposit for decreasing the rate of the “electron-hole” recombination’s occurring in the context of the photooxidation of pollutants and the photocatalytic splitting of water We have demonstrated that simple or successive incipient wetness impregnation followed by heating at 180oC is a simple and efficient method for dispersing effectively various amounts of C60 (in the range 1-4% w/w) on the titania surface used in photocatalysis The photocatalysts prepared were characterized using BET, XRD, DRS, and microelectrophoresis The supported C60 nanoparticles, comprised from 13-215 C60

molecules, have mean radius ranged between 0.8 and 3.1nm which increases with the C60

loading of the sample The supported phase was proved to be quite stable against sublimation/combustion

Chen Shifu et al [31] investigated p–n Junction photocatalyst NiO/TiO2 was prepared by sol–gel method using Ni(NO3)2·6H2O and tetrabutyl titanate [Ti(OC4H9)4] as the raw materials The p–n junction photocatalyst NiO/TiO2 was characterized by UV–vis diffuse reflection spectrum, X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) The photocatalytic activity of the photocatalyst was evaluated by photocatalytic reduction of Cr2O72− and photocatalytic oxidation of rhodamine B The results show that, for photocatalytic reduction of Cr2O72−, the optimum percentage of doped-NiO is 0.5% (mole ratio of Ni/Ti) The photocatalytic activity of the p–n junction NiO/TiO2 is much higher than that of TiO2 on the photocatalytic reduction of Cr2O72− However, the photocatalytic activity of the p–n junction photocatalyst NiO/TiO2 is much

lower than that of TiO2 on the photocatalytic oxidation of rhodamine B Namely, the p–n junction photocatalyst NiO/TiO2 has higher photocatalytic reduction activity, but lower photocatalytic oxidation activity Effects of heat treatment on the photocatalytic activity

of p–n junction photocatalyst NiO/TiO2 were investigated The mechanisms of influence

on the photocatalytic activity were also discussed by the p–n junction principle

I.7 The importance and direction of thesis

I.7.1 The importance of the thesis

Viet Nam is a developing country with a lot of investment projects The developing rate

of industry is high in the word The impact of the development is more and more serious polutted environment Wastewater not treated from companies is the cause of polutted river.Therefore, wastewater treatment, especially wastewater from coating companies contents a large amout of heavy metal, is an important task Nowaday, some methods are used to treat wastewater, in which, TiO2 are widely used to reduce heavy metals and degarade organic compounds from water In this paper, the degradations of heavy metal concentrations are investigated by using TiO2 as a photocatalyst and ethanol as a good electron suplier

I.7.2 The direction of thesis

According to Limin Wang and his co-worker, the photocatalytic reduction of Cr(VI) alone was dependent on both of specific surface area and crystalline structure of the photocatalyst in the absence of any organic compounds, but was dominated by the specific surface area of the photocatalyst in the presence of organic compounds because

of the synergistic effect between the photocatalytic reduction of Cr(IV) and the photocatalytic oxidation of organic compounds[6] So alumium oxide (Al2O3) with a large surface area is used as a carrying agent to synthesise a large surface area catalyst [35] In this thesis, TiO2/Al2O3 system was firstly studied systematically; the photocatalytic activities of samples in which TiO2 was loaded onto Al2O3 by several methods were tested and elements affected photocatalytic activities were carefully investigated It can be expected that if titanium oxide photocatalysts can be prepared on such alumium oxide, it would provide photocatalytic systems that would be highly efficient and reactive as compared to unsupported systems

CHAPTER II EXPERIMENTAL II.1 Reagents and materials

• Alumium oxide Al2O3 (commercial)(Merck-Germany), M=101.94 (g/mol)

• Titanium clorua TiCl4, (China), M= 189.71(g/mol), C=99%, d=1.726 g/cm3

• Titanium tetraisopropyl Ti(C3H7O)4 (Merck - Germany), M=284.22 (g/mol), C=98%

• Titanium oxide TiO2 (commecial) (P25), (Merck), M=97.866,

• Ethanol C2H5OH (China) M= 46 (g/mol), C= 99.97%

• Isopropanol C3H7OH (Merck-Germany), M= 60.1 g/mol, d= 0.786 g/ml

• Polyethylene glycol hexadecyl ether [2-[2-[2-hexadecoxyethoxyethoxy]ethoxy]ethoxy]ethoxy]ethoxy]ethoxy]ethoxy] ethoxy]ethoxy]ethoxy]ethoxy]ethoxy]ethoxy]ethoxy]ethoxy]ethoxy]ethoxy]ethoxy]ethanol, C16H33(OCH2CH2)nOH, n~10 (Brij 56) d=0.977(g/ml)

2-[2-[2-[2-[2-[2-[2-[2-[2-[2-[2-[2-[2-[2-[2-[2-[2-Tables II.1 Properties Computed from Structure:[48]

Molecular Weight 1123.49176 [g/mol]

• Nitric acid HNO3 (Merck-Germany), M=63.012 g/mol , C=65 ÷ 70%, d= 1.5129 g/ml

II.2 Preparation of photocatalysts

II.2.1 Sol-gel method

a Introduction:

A number of methods have been employed to fabricate TiO2, including codeposition, chemical vapor deposition, and sol–gel process However, the sol–gel process is one of the most appropriate technologies to prepare titanium oxide The interest in application of sol–gel method is due to several advantages including: good homogeneity, ease of composition control, low processing temperature, large area coatings, low equipment

cost, and good photocatalytic properties [15, 24]

b Application in this thesis

Ethanol (99.97 %)-China was used as a solvent; pH of solution was controlled at 2 by nitric acid; the temperature of solution was 400C and was maintained during gelation process 0.02 mole of Brij 56 was slowly dissolved in 120 ml of the above solution within 3h under vigorous stirring Then, an exactly titania precursor, TTIP (Merck) was slowly added to achieve the molar ratio of the compound, which was optimized at Brij56: TTIP

= 1:5 with respect to catalytic homogeneity and porous structure

The solution was stabilized for 1h at 400C under stirring Then, Al2O3 was added to achieve the mass ratio of TiO2/Al2O3 was 30% The suspension was stable for 15 minutes The temperature was increased to 600C -800C to evaporate ethanol The synthesized gel is transparent and highly viscous The final gel was dried at 1200C for 2h and calcinated at 6000C for 5h with the heating rate of 30C/min This catalyst was marked SG-30

Fig II.1 The Sol-gel method II.2.2 Hydrolysis method

The catalyst was synthesised in accordance with the following process TiCl4 was slowly added to distilled water (volume ratio 2:100; 4:100; 6:100; 8:100; 10:100) at room temperature and was marked HD 6; HD 12, HD 22; HD 30; HD 36, respectively The hydrolysis reaction was highly exothermic and produced high quantities of fumes of HCl After about 10 hours of continuous stirring a clear solution was obtained.10 g of Al2O3

were added to the TiCl4 solution and the obtained suspension was boiled for 2 hours in a two necks balloon with a pipe cooler The suspension was dried in a glass of 500 ml

under stirring at 80oC The final suspension was dried at 120oC for 2h and then, calcinated at the various temperatures (200oC, 400oC, 550oC, 700oC) for 3h with a temperature ramp of 2oC/min The amount of TiO2 loading onto Al2O3 of synthesized catalysts was showed in table II.2

Table II.2 The amount of TiO2 loading onto Al2O3 of synthesized catalysts

All the samples synthesized by hydrolysis method were marked HD X (Y)

X: the amount of TiO2 loading onto Al2O3 (%)

Y: the calcinated temperature of the sample

Fig II.2 Hydroysis method II.2.3 Impregnating method

The synthesized catalyst implemented as in detailed description Alumium oxide (Al2O3) (typicaly 3.2 g) was dispersed in 100 ml of isopropanol, and titanium isopropoxide was